A relay valve shown and described in Japanese Patent No. 44-27163 is one example of the prior art type of a pressure control valve and will now be explained in greater detail with reference to FIG. 4 of the subject application.
As shown in FIG. 4, an air supply chamber is characterized by numeral 1 while an output chamber is characterized by number 2. An exhaust chamber is illustrated by number 3 and an air supply valve is depicted by number 4. Further, it will be seen that an exhaust valve rod is depicted by numeral 5 and a piston is characterized by numeral 6.
In viewing FIG. 4, it will be observed that the air supply chamber 1 has an air supply passage 7 which is connected to the output chamber 2. A connecting port 8 is connected to a suitable compressed air reservoir via a pipe or conduit 8a. A valve seat 9 circumscribes the air supply passage 7 and projects upwardly from the lower side of the air supply chamber 1.
The output chamber 2 has a connection port 10 which is connected to a brake cylinder or the like, and also has an equalizing passage or hole 12 which is connected to a balance chamber which is disposed above the movable piston 6.
The exhaust chamber 3 is open to the atmosphere via the port 13.
The air supply valve 4 is located in the air supply chamber 1 and the upper reduced portion of it is in the back chamber 14 so that it can slide freely in the vertical direction to open and to close the air supply passage 7. The valve 4 is urged downwardly by a biasing or compression spring 15 which is disposed in the back chamber 14 so that it normally causes the valve 4 to seat on the valve seat 9. As shown in FIG. 4, an equalizing passage or hole 16 is located in the lower wall of the valve 4.
As shown, the exhaust valve rod 5 extends through the output chamber 2, through the exhaust chamber 3, and to the balance chamber 11. The upper flared rim or tip 17 of the exhaust valve rod 5 faces the underside of the air supply valve 4. Thus, the outside diameter of rod 5 is designed so that it forms the air supply passage 7. The rod 5 penetrates the wall dividing chambers 1 and 2 and slides freely there between but is air tight by suitable sealing rings. The enlarged piston portion 6 is located at the lower end of rod 5. The rod 5 has a central internal exhaust passageway 18 which extends from upper open end of the tip portion 17 to an opening formed at the other end leading to the exhaust chamber 3.
An enlarged main part 6a of piston 6 has a flange shape portion formed on the lower end of the exhaust valve rod 5. The inner edge of a resilient diaphragm 19 is attached on the outer periphery of the piston main part. The outer edge of the diaphragm 19 extends outwardly and is fixedly attached to the inner surface of the inside wall of the main body of the valve. The upper side of the piston 6 and diaphragm 19 form the above-mentioned balance chamber 11 and the lower side defines a command chamber 20. There is a return spring 21 in control chamber 20 which pushes the piston 6 toward the balance chamber 11. The control chamber 20 has a connection port 22 which connects to an air control supply exhaust pipe 22a.
In this pressure control valve, the condition shown in FIG. 4 is in an overlap state. In the overlap state, the upper tip 17 of the exhaust valve rod 5 is in intimate contact with the air supply valve 4 while the air supply valve 4 is seated on the valve seat 9. In other words, it is the condition in which the output chamber 2 is blocked off from the air supply chamber 1 and also in which the output chamber 2 is blocked off from the exhaust chamber 3.
In this overlap condition, the control force with which the control air pressure P1 in the control chamber 20 pushes the piston 6 upwardly is P2.times.S1, and the balance force with which the output air pressure P2 in the output chamber 2 pushes the piston 6 downwardly is P2.times.S2. The above-mentioned S1 is the effective area of the lower surface of the piston 6 and diaphragm on which the control pressure P1 in the air chamber acts, and S2 is the effective area of the upper surface of the piston 6 and diaphragm 119 on which the output air pressure P2 acts. When the force exerted by the return spring is F, the following equation is valid: EQU P2.times.S2+F=P1.times.S1
Since F is small, the output air pressure can be described by the following equation: EQU P2=(S1/S2).times.P1
In other words, the output air pressure P2 is the product of the control air pressure P1 and the effective area ratio of both sides of piston 6.
In this overlap condition, when the control air pressure P1 decreases, the control force becomes less than the balance force, and the piston 6 moves downwardly so that the tip 17 of the exhaust valve rod 5 is unseated from the air supply valve 4, the output chamber 2 connects to the exhaust chamber 3 via the exhaust opening 18. Thus, the output air pressure P2 decreases as a result of this exhausted condition so that the balance force decreases. Now when the balance force is equal to the control force, the valve returns to the overlap condition again. When the control air pressure P1 is reduced to atmospheric pressure, the output air pressure P2 is also reduced to atmospheric pressure.
In addition, in the overlap condition illustrated in FIG. 4, when the control air pressure P1 is increased, the control force becomes greater than the balance force, and the exhaust valve rod 5 pushes the air supply valve 4 upwardly to unseat it from the valve seat 9. In this manner, the air is supplied from the air supply chamber 1 to the output chamber 2 through the air supply passage 7. As a result of this air supply motion, the output air pressure P2 rises and the balance force also increases. When the balance force increases and is equal to the control force, it returns to the overlap condition.
Thus, in the pressure control valve illustrated in FIG. 4, the control air pressure P1 is changed so that a corresponding output air pressure P2 can be obtained. The output air pressure P2 may be used, for example, to operate a vehicle brake system.
In the pressure control valve of FIG. 4, there is only one piston 6 which is separated into a control piston on which the control air pressure P1 acts and which operates as a balance piston on which the output air pressure P2 acts.
The pressure control valve of FIG. 4 is designed so that the ratio of S1/S2 in equation P2=(S1/S2).times.P1 becomes constant. However, the characteristic of the output air pressure P2 to the control air pressure P1 may be changed depending on the type of air brake system. In other words, one in which the effective area ratio S1/S2 of the piston is different as required. In such a case, it can be managed by changing one of the effective areas S1, S2 in the pressure control valve of FIG. 4, but in reality, it is very inconvenient to change the design and to have to manufacture it individually for each particular application.
The prior art includes another method to change the effective area ratio of S1/S2, namely the one illustrated in FIG. 4 of the Japanese utility Model No. 61-2119. In this latter arrangement, there is an equivalent to the above-mentioned piston 6 which takes the form of the balance piston and the control piston, and a lever mechanism consisting of the lever and the fulcrum roller is placed between the two pistons, and there is a method to adjust the position of the fulcrum roller. In this structure, the lever ratio can be changed by changing the position of the fulcrum roller so that the size of the force transmitted changes, and, it therefore achieves practically the same result as in the case which the effective area ratio are changed.
In the latter mentioned pressure control valve, the structure of which includes the lever mechanism, the characteristic of the output air pressure P2 to the control air pressure P1 can be changed by adjusting the position of the fulcrum roller. However, the member in the axial direction of the piston provided between the lever and the piston is inclined slightly due to the rotation of the lever during the operation. Thus, it becomes difficult to transmit the work force precisely, and/or the part which affects the function, such as the part of that member which contacts the lever. Namely, the contacting part of the fulcrum roller and the lever tends to become worn so that even if the fulcrum roller is placed at the same position, the output air pressure to the defined control air pressure will be different from the original initial pressure. Thus, after it has been in use for a long time a decrease in sensitivity and response is a problem.